The present invention relates to a transducer circuit and a control circuit for use with the transducer circuit. The transducer circuit has particular, but not exclusive, relevance to metrological instruments for measuring surface characteristics such as form, for example roundness, and roughness or surface texture.
WO 95/08096 describes such a metrological instrument, the Form Talysurf Plus manufactured by Taylor Hobson Limited, Leicester, England, UK. In this instrument, a stylus is pivotally mounted on an arm and the tip of the stylus is traversed across the surface of a workpiece. A transducer outputs a signal in accordance with the movement of the tip of the stylus in response to a surface characteristic, in the case of the Form Talysurf Plus the surface roughness or texture, of the workpiece.
The transducer of the Form Talysurf Plus comprises an inductance which has a centre tap connected to earth and a moveable core which moves in response to movements of the stylus tip. A bridge circuit is formed by connecting each end of the inductance to a respective end of a variable potentiometer whose wiper is connected to earth.
An oscillator, whose centre tap is grounded, supplies an oscillating voltage via matched resistors to the bridge circuit. The voltages at the ends of the inductance are sampled and processed through a summing circuit and a difference circuit. When the bridge circuit is balanced, the output of the summing circuit is a null signal as the voltages at opposite ends of the inductance have identical amplitudes but are 180xc2x0 out of phase. If the bridge circuit is not balanced, for example due to movement of the core in response to movement of the stylus, the summing circuit will provide a non-null output signal. The output of the difference circuit is an oscillating signal with a substantially constant amplitude.
A digital output representative of the position of the stylus is obtained by:
i) supplying the output of the difference circuit to a multiplying digital-to-analogue converter (DAC) which multiplies the output of the difference circuit by a digital value;
ii) comparing the output of the DAC with the output of the summing circuit; and
iii) adjusting the value by which the DAC multiples the output of the difference circuit so that the output of the DAC has an identical amplitude and is 180xc2x0 out of phase with the output of the summing circuit.
The digital value by which the multiplying digital-to-analogue converter multiplies the output of the difference circuit provides a digital signal representative of the position of the stylus.
WO 95/08096 discloses how to adjust the circuit described above to reduce a number of errors which may be present in the measurement. One particular adjustment involves incorporating a variable phase shift network between the difference circuit and the multiplying DAC. This variable phase shift network can be adjusted to allow for phase shift between the outputs of the summing circuit and the difference circuit.
The variable phase shift network is automatically controlled by monitoring a signal obtained by summing the output of the summing circuit and the output of the multiplying DAC. If the outputs of the summing circuit and the difference circuit are out of phase there will be a residual ripple voltage when these two signals are added together. The phase shift circuit is automatically adjusted to minimise this ripple voltage thereby ensuring that the signals are substantially in phase.
Although the instrument disclosed in WO 95/08096 provides accurate measurement of surface characteristics, the instrument is also expensive.
The present invention is concerned with developing alternative instruments to that disclosed in WO 95/08096. In particular, the present invention aims to provide an instrument which can be manufactured at a significantly reduced cost compared to the cost of the instrument disclosed in WO 95/08096.
In one aspect, the present invention provides a metrological instrument, for example an instrument for measuring surface form and/or roughness, or a transducer circuit suitable for use in such an instrument, wherein a position transducer such as, for example, an LVDT (Linear Variable Differential Transducer) is provided with a digitally generated excitation signal.
In one aspect, the present invention provides a metrological instrument, for example an instrument for measuring surface form and/or roughness, or a transducer circuit suitable for use in such an instrument, wherein in order to ensure that the output of a position transducer, for example an LVDT (Linear Variable Differential Transducer), is sampled at a known point, for example at a known phase of the output signal, the output signal is compared with a number of samples corresponding to an excitation signal (which may be a digitally generated signal) supplied to the position transducer but each of different phase from one another to determine which of the samples is closest in phase to the output signal of the position transducer.
In one aspect, the present invention provides a metrological instrument, for example an instrument for measuring surface form and/or roughness, or a transducer circuit suitable for use in such an instrument, wherein in order to identify a peak in the output of a position transducer, for example an LVDT (Linear Variable Differential Transducer), a number, generally two, of points of the output signal having a known phase relationship with one another are sampled, by, for example, sampling the output signal when an excitation signal (which may be a digitally generated signal) supplied to the position transducer has first and second given phases.
In an embodiment, the present invention provides a transducer circuit, comprising: a sub-circuit for generating a periodically varying signal for supply to a position-to-electrical transducer, the position-to-electrical transducer being arranged to produce an analogue electrical signal dependent upon the position of a probe; an analogue-to-digital converter for converting the analogue electrical signal to a digital electrical signal; and a processor for analysing signals received from the analogue-to-digital converter and the sub-circuit to provide a measurement signal for providing a measurement of the position of the probe, wherein the sub-circuit includes digital components arranged to generate the periodically varying signal in accordance with a clock signal.
An advantage obtained by using a digitally based system to generate the periodically varying signal is that the properties of the periodically varying signal generated are well-defined and predictable and so it is not necessary to continuously monitor the properties of the periodically varying signal.
The replacement of the analogue oscillator of WO 95/08096 with a digitally based system can also lead to a reduction of the total number of components required in the transducer circuit as an increased amount of signal processing can be carried out in the processor due to the well-defined signal produced. There is also a reduction of noise due to the removal of analogue components which have an inherent noise and are susceptible to electrical pick-up.
In an embodiment, the present invention provides a transducer circuit, comprising: a clock producing a train of clock signals at a constant frequency; a sub-circuit for providing an analogue signal, whose voltage varies periodically, to a transducer for producing an electrical signal dependent on the position of a probe; an analogue-to-digital converter for converting the electrical signal into a digital signal; and a processor for analysing signals received from the sub-circuit and the analogue-to-digital converter to provide a measurement signal providing a measurement of the position of the probe, wherein the processor is adapted to correct for any phase shift between the analogue signal and the electrical signal in accordance with information received from the sub-circuit and the analogue-to-digital converter.
Preferably, the clock is a crystal oscillator as a crystal oscillator provides a signal which is substantially immune to frequency drift. Furthermore, a crystal oscillator does not require a long warm-up period.
Preferably the crystal oscillator is an integral part of the processor thereby reducing the total number of components in the circuit.
Conveniently, the amplitude of the analogue signal varies sinusoidally.
In an embodiment of the present invention, the transducer circuit is provided in a metrological instrument for measuring a characteristic of a surface of a workpiece, wherein the probe is a stylus and the position-to-electrical transducer is an inductive transducer having a coil and a core for the coil, the inductive transducer being arranged so that movement of the stylus in response to the characteristic of the surface causes relative movement between the coil and the core, and the instrument further comprises an arm for holding the stylus so that the stylus may be traversed relative to the surface of the workpiece.
A transducer circuit embodying the present invention is particularly well suited to metrological instruments of the type described above as high resolution can be obtained over a large measurement range.